Field of the Invention
The present invention relates to an evaporation cycle of a natural circulation steam generator. In more detail, the invention relates to an evaporation cycle of a natural circulation steam generator in connection with a vertical duct for upward gas flow, comprising a steam drum for feeding water to a downcomer pipe, an evaporator in flow connection with the downcomer pipe and comprising a first evaporative section and a second evaporative section connected in parallel with the first evaporative section and arranged at a higher level than the first evaporative section, each of the evaporative sections comprising multiple in parallel connected evaporation tubes comprising one substantially horizontal pass or multiple in series connected substantially horizontal passes across the vertical duct for evaporating the water to a mixture of steam and water, and a riser pipe in flow connection with the evaporator for conveying the mixture of steam and water to the drum.
Description of Related Art
A heat recovery steam generator (HRSG) can be arranged in connection with a horizontal gas flow duct or a vertical gas flow duct. The latter type, so-called a vertical HRSG, is generally advantageous, especially due to a small footprint area, efficient heat transfer, and relatively easy cleaning. A disadvantage of a conventional evaporation cycle in a vertical HRSG is that, due to horizontally extending evaporation tubes, usually, a circulation pump or other external source for a motive force is required to ensure required water flow through the evaporation tubes.
Each evaporation tube of a vertical steam generator generally comprises multiple in series connected passes across the gas duct, which collectively create a water flow that can be either parallel or countercurrent to the flow of the gas. This means that, assuming that the gas flow is upwards, the water flow is parallel to the gas flow if each in water flow direction latter horizontal pass is arranged downstream of, i.e., at a higher level than a respective in water flow direction earlier horizontal pass. Correspondingly, the water flow is countercurrent to an upward flow of gas if each in water flow direction latter horizontal pass is arranged upstream of, i.e., at a lower vertical level than a respective in water flow direction earlier horizontal pass.
European patent publication EP 0 764 813 B1 discloses an evaporation cycle of a heat recovery steam generator, without a circulation pump, comprising horizontal evaporation tubes in countercurrent flow to the hot gas stream, in which the inlet header of the evaporative section is connected to a downward loop of a downcomer pipe. This is a very simple cycle, but may not provide sufficient water circulation in all circumstances.
European patent publication EP 0 357 590 B1 and U.S. Pat. No. 5,575,244 disclose different, more complicated solutions for a heat recovery steam generator comprising horizontal evaporation tubes connected parallel to the gas flow, based on using an ejector to start natural circulation in the evaporation cycle. European patent publication EP 0 752 556 B1 discloses correspondingly a heat recovery steam generator with horizontal evaporation tubes, in which a stream of feedwater, flowing by the aid of a feedwater pump, is injected to the inlet side of the evaporation tubes to start the natural circulation.
U.S. Pat. No. 5,762,031 discloses a complicated heat recovery steam generator comprising horizontal evaporation tubes connected in parallel to the gas flow. In the steam generator, the evaporator is divided into two sections, the first one of which being directly connected to the feedwater line of the system, and the water flow therein is thus based on forced circulation with a feedwater pump. The second evaporative section is primarily based on natural circulation from a steam drum, but the outlet sides of the two evaporative sections are connected together so as to assist the natural circulation with the feedwater pump.
U.S. Pat. No. 1,486,888 discloses a steam boiler comprising a first evaporative section with inclined or partly inclined and partly vertical tubes for absorbing radiation heat in a vertical combustion chamber with upward gas flow, and a second evaporative section for absorbing convection heat with inclined tubes arranged across multiple sections of a gas path. French patent publication FR 1 523 735 A discloses a steam boiler having an evaporative section with evaporation tubes with an inclined section between horizontal end sections in a chamber for horizontal gas flow. Great Britain patent publication GB 810 900 A discloses a natural circulation steam generator comprising conventional evaporative tube bundles with multiple horizontal passes across a vertical gas flow in a pressure vessel.
An object of the present invention is to provide a simple, efficient, and reliable evaporation cycle of a natural circulation steam generator in connection with a vertical duct for upward gas flow.
A further object of the present invention is to provide an evaporation cycle of a natural circulation steam generator in connection with a vertical duct for upward gas flow, in which the evaporation cycle is not associated with an external source of motive force to assist the flow of the water in the evaporator.